Neat, non-esterified vegetable oils are often used as alternative, renewable, locally produced, low greenhouse gas emissions fuel for diesel engines, which are typically fitted with a heated secondary fuel system, and are started, warmed up, and shut down on diesel fuel. This paper addresses the question of the temperature to which the fuel should be, can be, and is heated. Experiments done on a tractor engine with a mechanical inline injection pump revealed that at sustained higher loads, the fuel can be heated by engine coolant nearly to the thermostat opening temperatures prior to the injection pump inlet, with additional heating taking place before the fuel reaches the injector inlet. While vegetable oil heating to at least about 40-50°C was beneficial to prevent large power loss on a common-rail type engine tested, excessive heating decreased the maximum engine torque on the engine with an inline injection pump, and accelerated degradation of the fuel. At sustained idle, the coolant and fuel temperature decrease, with additional cooling of the fuel by the time it reaches the injector, and the delay of the onset of the combustion increases with decreasing fuel temperature. Recycling of the return fuel line back to the inlet is both beneficial, as it accelerates the heating of the fuel during engine warm up, prevents excessive heating of the fuel in the fuel tank, and prevents heated and possibly degraded fuel from returning to the tank, but also has drawbacks, as some heating of the fuel tank by the returned fuel is needed to ensure adequate fuel flow and filterability. Design of the secondary fueling system should therefore consider the engine type and the anticipated operating conditions.